1. Field of the Invention
Embodiments of the present invention relate generally to an airway adapter and a gas analyzer for measuring oxygen concentration of a respiratory gas.
2. Description of the Prior Art
In anesthesia or in intensive care, the condition of a patient is often monitored by analyzing the gas exhaled by the patient for its content. For this reason either a small portion of the respiratory gas is diverted to a gas analyzer or the gas analyzer is directly connected to the respiratory circuit. The former analyzer is known as a sidestream sensor, while the latter is known as a mainstream sensor because of its ability to measure directly across the respiratory tube. Typically, a mainstream sensor has a disposable airway adapter and a directly connectable sensor body. The majority of mainstream sensors on the market are designed to measure carbon dioxide alone, using an infrared non-dispersive (NDIR) absorption technique. As it is not directly related to this case, NDIR measurement will not be further described in this document.
Another gas of vital importance is, of course, oxygen. Oxygen can be measured using chemical sensors or fuel cells, but they are normally too bulky to fit into a mainstream sensor and, although they have a limited lifetime, they are not designed for a single use and must therefore be protected from direct contact with the patient gas to avoid contamination. This is expensive and also influences the response time of the sensor. Oxygen can also be measured using a laser at an absorption of 760 nm. However, this absorption is very weak and the signal from the short distance across the respiratory tube is too noisy to be useful. The most promising method is luminescence quenching. A special sensor coating, a luminophore, is excited using, for example, blue light from a light emitting diode (LED). A luminescence signal can be detected at longer wavelengths, often in the red portion of the spectrum. Oxygen has the ability to quench this luminescence in a predictable way by consuming the available energy directly from the luminophore. Thus, the amount of quenching is a direct measure of the partial pressure of oxygen in the respiratory gas mixture. Luminescence quenching offers the possibility to make a single use probe in connection with the patient adapter. However, problems associated with luminescence quenching relate to temperature and humidity dependence as well as drift caused by aging. Also, the luminescence intensity is not normally measured directly. Instead a change in the decay time of the excited state is a more stable and robust measurement. Still, an optical reference is normally a necessity, as is temperature compensation.
If carbon dioxide and oxygen can be measured reliably, it is possible to calculate oxygen consumption and carbon dioxide production of a patient, provided the respiratory flow is also known. The flow can be measured using a hot wire technique, but because of the difficult environment with water and mucus, it would require filtration of the respiratory gas. This again increases the flow resistance. A better method utilizes the pressure drop that develops across an obstacle in the gas stream or a pressure signal from a pitot tube. An interesting method is based on the behavior of vortexes formed downstream of a bluff body. The vortex flow meter has a large dynamic range and is fairly linear and robust. It can be based on vortex frequency estimation or vortex time of flight estimation. As will be evident below, the real advantage comes from the fact that the oxygen sensor can reside in the bluff body.
In the clinically used gas analyzer of a mainstream sensor, the whole volume, or at least the main portion of the breathing air or gas mixture, flows through the analyzer and its disposable measuring chamber. Because the measuring chamber is in the breathing circuit, it is easily contaminated by mucus or condensed water. Thus, it is necessary to use sensors that are as robust and insensitive to the conditions as possible. The infrared sensor uses one or more reference wavelengths in a mainstream analyzer in order to provide a good enough estimate of the signal level without gas absorption, the zero level, continuously available. For the oxygen sensor, it is important that contamination does not alter the sensitivity more than can be tolerated. The sensor based on luminescence quenching fulfills this demand. It is known that it works submerged in water as it measures the dissolved oxygen. The response time will naturally be longer in such a measurement. As already mentioned, the flow sensor must also tolerate contamination. In this respect the sensor based on vortex formation seems to be reliable.
A clinical mainstream gas analyzer must be small, light, accurate, robust and reliable. It is not possible to make a zeroing measurement using a reference gas during its normal operation. Yet, the analyzer must maintain its accuracy even if the measuring chamber is contaminated. Due to these requirements, mostly single gas mainstream analyzers for carbon dioxide (CO2) have been commercially available. A compact CO2 and O2 gas analyzer with flow sensor of the mainstream type has been technically very challenging.
Another requirement is that the measurement be fast enough to measure the breathing curve. In practice, the rise time would have to be in the order of about 200 ins or even shorter. For CO2, this is possible to arrange using well known infrared measuring technique. The luminescent O2 sensor, however, must have a very thin layer of active material in order to react fast enough. This decreases the signal, and to compensate, the sensor surface must be increased. For a small mainstream sensor with a luminophore coated window for measuring light transmission, this may be a problem. Regarding the flow sensor, it can be arranged to be fast if the related sensors are fast, so the response time is not a problem technically.